Hi,

I have 4 Smoothwall boxes (Linux 2.4, IPTABLES, 2 NIC's) and I need
the first box to re-route traffic to the other Smoothwall boxes
depending on the source IP address (basically so it just becomes
another hop in route). I have spent hours trying to get it to work
using examples and HOWTO's but have had no luck what so ever.

I have already spent over a week trying to get our L3 routing switches
to do this but HP tell me it is impossible, this leaves me with
Smoothwall and IPTABLES. They are all Corporate Server 3 boxes (but
they are very similar to GPL v2.0) and I have also tried asking
Smoothwall directly but the response was that it "might" affect the
effectiveness of Smoothwall.

Here is a diagram of the setup...

http://www.niceandsafe.f2s.com/smooth/srcroute.gif


I have tried the following lines in my rc.firewall.up file without success

/sbin/iptables -t nat -A POSTROUTING -o eth0 -s 10.1.2.0/24 -j SNAT --to 10.1.1.11

OR

/sbin/iptables -t filter -I FORWARD -i eth0 -o eth0 -s 10.1.2.0/24 -j
ACCEPT -d 10.1.1.11


I know I don't have to specify the table 'filter', this is just for
the example, and I also know that something is probably very wrong
here but I have never used source based routing before, or IPTABLES
for that matter. And yes, eth0 is definitely my Green interface.

thanks in advance

First of all, this isn't easily done with netfilter, but it can be
done using the ROUTE target:
http://www.netfilter.org/documentation/pomlist/pom-extra.html#ROUTE

You'd need to do something like:
iptables -A POSTROUTING -t mangle -s 10.1.2.0/24-j ROUTE --gw 10.1.1.11
iptables -A POSTROUTING -t mangle -s 10.1.3.0/24-j ROUTE --gw 10.1.1.12

It can be done with no kernel modification, by using iproute2's policy routing. 
First, you need to create two (Or three, you only mentioned two VLANs
in your diagram, but there were three listed.) tables:

echo 200 VLAN2 >> /etc/iproute2/rt_tables
echo 201 VLAN3 >> /etc/iproute2/rt_tables

For VLAN2 (VLAN3 will be similar):

ip route add 10.1.1.0/24 dev eth0 table VLAN2 
ip route add default via 10.1.1.11 table VLAN2
ip rule add from 10.1.2.0/24 table VLAN2

In English, that sets up an alternate routing table for all packets
coming from 10.1.2.0/24, with a route for 10.1.1.0/24 through eth0 and
a default route through 10.1.1.11.
If for whatever reason you want packets coming back from the Internet
to go through 10.1.1.10, you need to, on Smoothie2, add the following
route (This applies to the netfilter method, too):

ip ro add 10.1.2.0/24 via 10.1.1.11

For more details about this, check out
http://lartc.org/howto/lartc.rpdb.html#LARTC.RPDB.SIMPLE

I strongly recommend the second method though, not only because it's
the only one I've tried, but because it keeps routing decisions where
they should be, not in the packet filter.

HTH

ip ro add 10.1.2.0/24 via 10.1.1.11 should, obviously, read:
ip ro add 10.1.2.0/24 via 10.1.1.10